All product should be documented. Particularly when any release happens product must have release notes because release notes educates people about product and its usage.
Microsoft has also release notes for SQL Server 2008. This Release Notes document contains information for Microsoft SQL Server 2008 Release Candidate 0 (RC0) that supplements the SQL Server 2008 RC0 Readme and Books Online documentation.
Abstract courtesy : Microsoft
Reference : Pinal Dave (http://blog.SQLAuthority.com)